1. Field of the Invention
The present invention relates to a wet-type multi-plate clutch used in a clutch and/or a brake of an automatic transmission (AT) of a vehicle. More particularly, the present invention relates to an improvement in friction surfaces of an internally-toothed plate and an externally-toothed plate.
2. Description of the Related Art
In the past, wet-type multi-plate clutches were provided with separator plates as externally-toothed plates and friction plates as internally-toothed plates obtained by adhering friction materials on core plates and were designed to transmit torque by engaging the externally-toothed plates with the internally-toothed plates.
Further, most of friction heat generated by these plates upon engagement of the clutch is transferred to the externally-toothed plates or the internally-toothed plates to which the friction materials are not adhered to be dispersed, since the friction material has a predetermined adiabatic property.
In the past, a wet-type multi-plate clutch comprising friction plates as internally-toothed plates on both surfaces of which friction materials are adhered, as shown in FIG. 1 of Japanese Patent Application Laid-open No. 11-141570 (1999), has been known. Further, a plate arrangement of one-surface alternately adhering type in which the friction materials are adhered on surfaces of the internally-toothed plates and the externally-toothed plates which are not opposed to each other, respectively, is also known.
Although a plate thickness of the externally-toothed plate can be increased to increase a dispersed heat amount in order to enhance the adiabatic property, if the plate thickness is increased, an axial length of the plate is also increased, thereby arising a new problem that the clutch cannot be designed to have a compact size. To avoid this, there has been proposed a clutch of one-surface alternately adhering type in which the friction materials are adhered alternately on one-surfaces of the externally-toothed plates and the internally-toothed plates so as to disperse the friction heat from both of the externally-toothed plates and the internally-toothed plates.
However, in the clutch comprising the plates of one-surface alternately adhering type, since the friction heat is generated only on the single surface of each plate, unbalance of heat is caused between both surfaces of each plate to deform the plate. Consequently, a bad influence will be affected upon the endurance and the friction property of the friction material. Further, during an idle rotation of the clutch, since axial loads are imbalanced between front and rear surfaces of the plate, there arises a problem that it is difficult to maintain a clearance between the internally-toothed plate and the externally-toothed plate constant, thereby increasing idle rotation drag torque.